We have "stockpiled" milligram quantities of partially purfifie estradiol 178-dehydrogenase from human placents. This will be purified by affinity chromatography. We intend to crystallize the enzyme (and pilots have already been successful). We will continue study of the binding site with 4-bromoacetamidoestradiol and 2- bromoacetamidoestradiol. Difference spectroscopy will be carried out to see i this technique indicates that binding of steroid perturbs those chromophoric amino acid residues that are alkylated by the affinity labeling steroids. Calf extradiol receptor will be purified by affinity chromatography and the steroid binding site studied with affinity labeling estrogens. The amino acid residues of the 20 beta-hydroxysteroid dehydrogenase adjacentto those affinity labeled by the 2,6,11,16 and 21 bromoacetoxy progestins will be elucidated by affinity labeling, specific hydrolysis by proteases, and sequencing of carboxymethylated peptides. Bibliographic references: S. W. Clark, F. Sweet, J.C. Warren: Synthesis and Use of Affinity-labeling Steroids for Interceptive Purposes. Am. J. Obstet. Gynecol. 121:864, 1975; S.W. Clark, F. Sweet & J.C. Warren: Interceptive Activity of 16 alpha-Bromo- acetoxyprogesterone. Biology of Reproduction 11:519, 1974.